Complimentary Metal Oxide Semiconductor (CMOS) Active Pixel Sensors (APS) are increasingly being used as a lower cost alternative to Charge Coupled Devices (CCDs). A CMOS image sensor circuit includes a focal plane array of pixel cells, each one of the cells includes a photo-conversion device, such as a photogate, photoconductor, or photodiode, for accumulating photo-generated charge. Each pixel cell may include a transistor for transferring charge from the photo-conversion device to a sensing node, and a transistor, for resetting a sensing node to a predetermined charge level prior to charge transfer. The pixel cell may also include a source follower transistor for receiving and amplifying charge from the sensing node and an access transistor for controlling the readout of the cell contents from the source follower transistor.
In a CMOS image sensor, the active elements of a pixel cell perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) transfer of charge to the sensing node accompanied by charge amplification; (4) resetting the sensing node to a known state before the transfer of charge to it; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing pixel charge from the sensing node.
CMOS image sensors of the type discussed above are generally known as discussed, for example, in Nixon et al., “256×256 CMOS Active Pixel Sensor Camera-on-a-Chip,” IEEE Journal of Solid-State Circuits, Vol. 31(12), pp. 2046-2050 (1996); and Mendis et al., “CMOS Active Pixel Image Sensors,” IEEE Transactions on Electron Devices, Vol. 41(3), pp. 452-453 (1994).
A conventional CMOS pixel cell 10 is illustrated in FIG. 1. The CMOS pixel cell 10 is a four transistor (4T) cell. Pixel cell 10 comprises a photo-conversion device, typically a photodiode 21, for collecting charges generated by light incident on the pixel cell. A transfer transistor 7, when activated by a transfer control signal TX, transfers photoelectric charges from the photodiode 21 to a sense node (charge to voltage conversion node), typically a floating diffusion region 3. A reset transistor 17 resets the floating diffusion region 3 to a predetermined voltage, Vaa_pix, in response to a reset control signal RST before the floating diffusion region 3 receives photo-generated charge. Floating diffusion region 3 is also electrically connected to the gate of an output source follower transistor 27. Pixel cell 10 also includes a row select transistor 37 for outputting a signal representing the charge on the floating diffusion region 3 from the source follower transistor 27 to an output column line 40 in response to an address signal SEL.
In the pixel cell 10 depicted in FIG. 1, the photo-conversion device generates and accumulates charge, e.g., electrons, in response to incident light during an integration period. The charge is transferred to the floating diffusion region 3 by activating the gate of the transfer transistor 7. The source follower transistor 27 produces an output signal from the transferred charges, which is readout via the column line 40.
A typical CMOS image sensor includes an array of pixels 10, arranged in a predetermined number of columns and rows. Generally, each pixel generates charge for a same amount of time. Such image sensors have a characteristic dynamic range. Dynamic range refers to the range of incident light that can be accommodated by an image sensor in a single frame of pixel data. It is desirable to have an image sensor with a high dynamic range to image scenes that generate high dynamic range incident signals, such as indoor rooms with windows to the outside, outdoor scenes with mixed shadows and bright sunshine, night-time scenes combining artificial lighting and shadows, and many others.
The dynamic range for an image sensor is commonly defined as the ratio of its largest non-saturating signal to the standard deviation of the noise under dark conditions. Dynamic range is limited on an upper end by the charge saturation level of the sensor, and on a lower end by noise imposed limitations and/or quantization limits of the analog to digital converter used to produce the digital image. When the dynamic range of an image sensor is too small to accommodate the variations in light intensities of the imaged scene, image distortion occurs.
As pixel size is scaled down, so is the size of the photo-conversion device. Therefore, the amount of charge the photo-conversion device and pixel cell can accumulate is reduced, degrading the sensor's dynamic range. There are several approaches to improve dynamic range, one of which utilizes dual integration periods. Another approach would be to add transistors to the pixel. Since it is difficult to implement additional transistors inside a pixel while at the same time maintaining a small pixel size (e.g., less than approximately 3.2 μm), the dual integration period approach is more desirable because the pixel cell can remain the same and only pulse timing related modifications are needed.
Orly Yadid-Pecht et al. in the article “Wide Intrascene Dynamic Range CMOS APS Using Dual Sampling,” IEEE Transactions of Electron Devices, Vol. 44, No. 10, pp. 1721-1723 (October 1997) (“Yadid-Pecht”) discloses a method of operating an image sensor where, for each frame, the output data includes two sets of pixel cell outputs each having different integration periods. The image sensor according to Yadid-Pecht requires additional column signal processing circuitry above the pixel cell array. Additionally, the methods disclosed by Yadid-Pecht are not well suited to color imaging.
Wandell et al., “Multiple Capture Single Image Architecture with a CMOS Sensor,” Chiba Conference on Multispectral Imaging, pp. 11-17 (1999) (“Wandell”) disclosed an imaging method using multiple integration periods. With the method described by Wandell, a CMOS image sensor includes one analog to digital converter (ADC) for each 2×2 pixel cell block. The pixel cells of each block are switchably connected to a storage capacitor, which in turn is connected to the ADC. Thus, the method of Wandell requires additional circuitry and cannot be implemented using conventional CMOS image sensor circuits, which is undesirable.
U.S. Pat. No. 6,008,486 to Stam et al. (“Stam”) discloses a method using dual integration periods. With Stam's method, each pixel cell integrates charge during a short integration period and a long integration period for each frame. If the pixel cell becomes saturated during the long integration period, a threshold detector causes the short integration period for that pixel cell to be used. Because each pixel cell integrates charge for two time periods per frame, additional sample and hold circuitry is required, which is undesirable.
What is needed, therefore, is a method for operating a CMOS image sensor to achieve an improved dynamic range, which can be implemented using conventional CMOS pixel cells. Additionally, it would be advantageous to have an improved method for operating a CMOS image sensor to achieve an improved dynamic range that is also well suited to color imaging.